Why is ISO considered part of the exposure triangle if it mainly changes gain?
Asked 6/19/2016
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I understand aperture and shutter speed directly affect how much light reaches the sensor, but ISO on a digital camera is often described as electronic amplification rather than a true change in sensor sensitivity. If ISO mostly changes gain after capture, why is it still treated as one side of the exposure triangle? Is it because exposure in practice refers to the brightness of the recorded image rather than only the physical amount of light collected?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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You're both right and wrong.
Yes, technically the "ISO setting" is merely an amplification of sensor data. However the quantization (feeding the analog signal into the analog-digital converter) happens after the amplification. So, from the sensor (as in photosensitive die alone) point of view, the amplification doesn't change the actual light sensitivity. But for the rest of the camera, "sensor" is the whole package: photosensitive silicon, amplifier and DAC. So, from the point of the camera, the effective sensitivity is changed.
Bonus: RAW is what comes straight from DAC. So even if you're dealing in RAWs, the "ISO setting" has already affected exposure.
Originally by user37063. Source · Licensed CC BY-SA 4.0
user37063
10y ago
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ISO is included in the exposure triangle because it changes the brightness of the recorded image and affects which aperture/shutter combinations will produce a usable result.
Strictly speaking, aperture and shutter speed control how much light is captured. ISO usually does not make the photosites themselves more sensitive; instead it applies amplification to the sensor signal before digitization. So at the sensor surface, exposure is still set by light, aperture, and time.
But in practical camera use, ISO changes the effective sensitivity of the imaging system. Raising ISO makes the image data brighter, changes how the camera meters and chooses shutter speed/aperture in auto and semi-auto modes, and affects dynamic range/noise tradeoffs. That is why photographers treat it as part of exposure control.
So the distinction is:
- physical exposure = amount of light collected
- image exposure in practice = brightness of the recorded result
Because ISO strongly affects the final recorded image and the settings needed to achieve it, it belongs in the exposure triangle even if it is not the same kind of control as aperture or shutter speed.
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